The consequence of leaf life span to virus infection of herbivorous insects

Many herbivorous insects die of pathogen infections, though the role of plant traits in promoting the persistence of these pathogens as an indirect interaction is poorly understood. We tested whether winter leaf retention of bush lupines (Lupinus arboreus) promotes the persistence of nucleopolyhedroviruses, thereby increasing the infection risk of caterpillars (Arctia virginalis) feeding on the foliage during spring. We also investigated whether winter leaf retention reduces viral exposure of younger caterpillars that live on the ground, as leaf retention prevents contaminated leaves from reaching the ground. We surveyed winter leaf retention of 248 lupine bush canopies across twelve sites and examined how it related to caterpillar infection risk, herbivory, and inflorescence density. We also manipulated the amount of lupine litter available to young caterpillars in a feeding experiment to emulate litterfall exposure in the field. Greater retention of contaminated leaves from the previous season increased infection rates of caterpillars in early spring. Higher infection rates reduced herbivory and increased plant inflorescence density by summer. Young caterpillars exposed to less litterfall were more likely to starve to death but less likely to die from infection, further suggesting foliage-mediated exposure to virus. We speculate that longer leaf life span may be an unrecognized trait that indirectly mediates top-down control of herbivores by facilitating epizootics.尽管植物性状作为间接互作因子促进这类病原体存续的机制尚未得到充分阐明，但诸多植食性昆虫均会因病原体感染而死亡。我们针对树生羽扇豆（Lupinus arboreus）的冬季留叶性状是否能够促进核型多角体病毒（nucleopolyhedrovirus）的存续，进而提升春季取食其叶片的灯蛾（Arctia virginalis）幼虫的感染风险展开了测试。此外我们还探究了冬季留叶是否会降低栖息于地面的低龄灯蛾幼虫的病毒暴露风险——这是由于留叶可阻止沾染病毒的叶片飘落至地面。我们在12个样地内对248株树生羽扇豆冠层的冬季留叶情况开展了调查，并分析了该性状与灯蛾幼虫感染风险、植食为害程度以及花序密度之间的关联。我们还通过室内取食实验，调控了低龄灯蛾幼虫可接触的羽扇豆凋落物量，以模拟野外环境中的凋落物暴露场景。上一季沾染病毒的叶片留存越多，早春时节灯蛾幼虫的感染率便越高。感染率升高会降低植食为害程度，并在夏季提升植株的花序密度。接触凋落物更少的低龄灯蛾幼虫更易因饥饿死亡，但因感染致死的概率更低，这进一步佐证了叶片介导的病毒暴露路径。我们推测，更长的叶片寿命可能是一种尚未被认知的植物性状，它通过促进昆虫流行病的发生，间接介导了植食性昆虫的下行调控效应。